Turbine bucket



1953 'r. N. HULL, JR 2,660,401

TURBINE BUCKET Filed Aug. 7, 1951 CROSS sEcr/a/v APE/7 .Iiilh a 221354;Inventor":

Th cm as N. Hull Jr.

A 8 8 by W His Attr-ney.

Patented Nov. 24, 1953 TURBINE BUCKET Thomas N. Hull, Jr., Schenectady,N. Y., assignor to General Electric Company, a corporation of New YorkApplication August 7, 1951, Serial No. 240,707

3 Claims.

This invention relates to turbomachine blades or buckets, particularlyto a turbine bucket for service in high temperature gas turbinepowerplants.

In recent years, in connection with the development of practicable gasturbine powerplants, there have been developed numerous modifications ofthe so-called vortex type of blade, characterized by a blade shape thatis warped or twisted from root to tip in order to obtain various desiredaerodynamic characteristics. In order to reduce the stresses in theblade due to centrifugal force, it has been customary to taper the bladefrom root to tip so that it has a continuously decreasing cross-sectionarea. Likewise, the transition in cross-section shape, and the change inentrance and exit angles, has been gradual from root to tip. Thisuniform transition along the length of the blade has been dictated bythe aerodynamic considerations involved in designing blades according tothe various vortex design theories.

As the capacity of gas turbine power-plants has progressively increased,it has been necessary to rapidly increase the length of the compressorand turbine blades, with the result that serious troubles have beenencountered with fatigue failures of the comparatively long slenderblades. These problems have resulted partly from the use of shroudlessbuckets, as contrasted with earlier steam turbine rotor structures inwhich a continuous shroud band of some sort is secured to the buckettips. Such shrouds have an important influence in reducing tangentialvibration of the buckets, which type of vibration appears to have a mostserious effect in producing fatigue failures. With a shroudless bucket,fixed more or less rigidly at its base and completely free to vibrate atits tip, it is extremely diflicult to so design a gas turbinepower-plant that the blades will not, at some speed or other, experienceexcessive resonant vibration due to discontinuities in the motive fiuidstream. These discontinuities may be caused by the fact that the motivefluid is produced by six or more separate combustors spacedcircumferentially around the axis of the power-plant, or by thepartitions in the turbine nozzle ring or struts extending across theflow path.

These fatigue failures of turbine buckets have been an important sourceofdifficulty in placing in regular commercial operation the first largegas turbine power-plants. It has become increasingly necessary to find asatisfactory and simple solution for this problem in order to make Allpossible large scale commercial exploitation of thi type of prime mover.

Accordingly, the object of this invention is to provide an improvedturbine bucket having a novel configuration especially designed toeliminate fatigue failures in the blade root or dovetail due to thelong, slender, shroudless bucket having a natural frequency so low as tobecome resonant under the influence of discontinuities in the motivefluid flow.

While improvement in the bucket vibration characteristics could beeffected merely by substantially increasing the axial width of thebucket, that expedient would proportionately increase the axialthickness and the weight of the turbine rotor. Accordingly, it is afurther object to effect this improvement in bucket fatigue strengthwithout increasing the axial thickness of the bucket-wheels.

Other objects and advantages will become apparent from the followingdescription, taken in connection with the accompanying drawings, inwhich Fig. l is a front perspective view of a turbine bladeincorporating invention, Fig. 2 is a side view of the same blade, Fig. 3is an end view looking at the tip of the blade, Fig. 4 is a sectionalview illustrating the shape of an intermediate portion of the blade atthe plane identified C in Fig. 2, and Fig. 5 is a graphicalrepresentation of certain design characteristics of the blade.

Generally, the invention is practiced by dividing the effective lengthof the blade into three portions, a root portion having a cross-sectionof substantial thickness and being shaped generally as an impulse typeof blade, a tip portion havin a very thin cross-section of airfoilshapeand constituting a reaction type of blade, and an intermediateportion which sharply transitions from the impulse blade shape at themotto the reaction type of blade shape at the tip.

Referring now more particularly to Fig. 1, the blade comprises aconventional base portion 1 provided with dovetail grooves la, or any ofthe many well-known equivalent means for fastening the base to the rimof the bucket-wheel. I he precise type of fastening is not material toan understanding of the present invention; and it will be appreciatedthat the novel blade shape which constitutes the present invention isapplicable to shroudless buckets or blades wherever the length of theblade is such that vibration problems, with resulting risk of fatiguefailures, are encountered.

As shown in Figs. 1 and 2, the blade 2 is di- 3 vided radially intothree portions of distinctive shape. These three portions are definedbetween the planes identified A, B, C, D in Fig. 2. The root portionimmediately adjacent the bucket base I extends from the plane A to theplane B. This portion is identified 2a, and is characterized by thecross section shape shown at 2.11 in Fig. 3. It will be. immediatelyapparent to those skilled in the turbine art that this blade shape isessentially that of the well-known impulse type of turbine, having asection of very substantial thickness, the maximum thickness occurring.approximately at the midpoint of the axial width of the blade. This typeof blade is roughly crescent-shaped; and, because of the cross-sectionshape and the substantial thickness, it has excellent resistance tovibration. in a tangential direction. The precise shape of this portionof the blade is of course determined in accordance.

with the well-known design principles governing the impulse. type ofblade, as used for many years past in steam turbines. For instance, theentrance angle may be on the order of 29 and the exit. angle may be onthe order of 28.

Attention is particularly directed to the fact that both the crosssection shape-of the blade and the cross section area are exactly, orvery nearly exactly, constant from the section A adjacent the base ithroughout approximately the root third of the blade, that is, extendingoutward to. the plane B in Fig. 2. In other words, the outlineidentified 2a in Fig. 3 represents the shape. oi the blade at bothplanes A and- B in Fig. 2.

The outer third of the blade is identified 2c in Fig. 2, and has. aairfoil-shaped crosssection indicated at 3: in. Fig. 3.. It willimmediately be apparent that this tip section is of, very much smallercross-section area than the root section 2a, also that the maximumthickness, in accordance with the. Well-known aerodynamic design ofairfoil sections, occurs substantially for- Ward of the middle of thechord; and the entrance angle has increased very substantially and thedischarge angle has decreased so that the outer third of the blade hasthe characteristic warped shape of blades designed in accordance withthe above-mentioned vortex theories. Specifically, the entrance angle atthe tip may be on the order of 80, and the exit angle may be about Themiddle third of the blade, identified 2b in. Fig. 2. iraneitions veryrapidly, yet'smoothly and continuously, from the impulse type root sec.-tion to the thin airfoil type tip section. To. indicate the rapidity ofthis change in shape, reference may be had to Fig. 4, which representsthe cross-section shape of the blade at the plane identified C in Fig.2. It will be apparent that this section is still of comparatively greatthickness, but that the shape has changed to that of an airfoil with thepoint of maximum thickness at about one-third the chord of the airfoilfrom the leading edge. The entrance angle is about 45 degrees and theexit angle about 25 degrees.

The manner in which the blade shape transitions from the impulse shapeat the root, as shown at 2a in Fig. 3, to the airfoil section 4 atsection C, may be seen from a comparison of the outlines of the blade asshown in Figs. 1 and 2. It will be seen that this transition in shapetakes place with great rapidity in the intermediate blade portion 2b.The corresponding change in crosasection area as a function of radius isillustrated graphic-ally in Fig. 5. Here the abscissa represents theradial length of the blade, with the stations A, B, C, D marked as inFig. 2. The curve identified 5 represents the change in cross-sectionarea from root to tip. It will be seen that the area is exactly, or verynearly exactly, constant throughout the root portion 2a (from plane A toplane B). The cross-section area is also substantially constant, but ata very much lower value, throughout the tip portion 20 (from plane C toplane D). And it. changes with great rapidity in the intermediate bladeportion 2b (from plane B to plane C). More specifically, it may be notedthat the crosssecti'on area in the tip portion is on the order of only:about 20 to 30 per cent of the root area between plane A and plane B.Generally, it may be stated that the. cross-section area of the tipportion is not more than about one-third that of thefroot portion.

It is also interesting to note the corresponding change in value of thetangential moment of inertia of the. cross-section area, which designcharacteristic has the most important effect on the resistance of theblade to vibration in a tan genti-al direction. This characteristic isrepresented by the curve labeled 6- in Fig. 5. It will be apparent thatthe tangential moment of inertia of the blade section follows generallythe shape of the cross section area curve, the moment of inertia at thetip being only on the order of a few per cent of the moment of inertiaof the root section 2a. More specifically, it may be stated that thetangential moment of inertia of the blade section throughout the tipportion 20 is not more than about one-tenth the moment of inertia at theroot portion 2a.

As described above, the root portion 2a is. en F stantially an impulsetype: blade, while the tip portion 20 is essentially a reaction typeblade. while the transition portion 212 changes. very rapidly from animpulse type section to substantially a reaction blade. Actually, it isnot necessary that the root portion 2a be a impulse section all the wayfrom section A to. section B. The blade section will be. ofsubstantially pure impulse typev at the section A; but the crosssectionshape may change slightly with radiusso that. at section B, the shapevis suitable. for perhaps 10 per cent reaction. Thus, the root portion 2ais for all practical purposes an -impulse type blade section.

As contrasted with the prior art blade shapes having a gradualcontinuous transition in both shape and cross-section area from root totip, the invention provides a blade which has, been found to haveunusual resistance to vibration in a tangential direction relative tothe bucketrwheel to which it is secured. Whereas blades of the samegeneral ,size and capacity designed according to previous gas turbinepractice had a fatigue life of only on the order of a few hundred thou,-sand cycles, corresponding to a few hundred hours of normal operation,tests of sample blades made in accordance with the invention haveindicated the life will be many years.

This improvement in fatigue resistance is achieved by the inventionwithout increase the axial thickness of the bucket-Wheels and,therefore, with substantially no change in the total weight of therotor. There may be some slight increase in the rotor weight, due. tothe fact that the root section of the blades is some.- what thicker thanthe prior art designs. The invention is also advantageous in that it,permits greater bucket length, for a given weight. of rotor and lengthof service life, so that the motive fluid flow path area can beincreased to decrease the velocity of the fluid leaving the turbine and,therefore, reduce the leaving loss represented by the velocity energyremaining in the motive fluid. Thus, an improvement in aerodynamicefiiciency can be obtained without increasing the weight of the rotor,and with much better life expectancy than with prior art blade shapes.

Stated another way, the advantage of the design is apparent from thefact that it has permitted a 40 per cent reduction in bucket width(corresponding generally to a 40 per cent reduction in bucket-wheelweight) over that which would. be required for the same fatigueresistance in a bucket not using the invention. This design alsoimproves the buckets resistance to vibration in the axial direction.

Thus, it will be seen that the invention provides a novel turbine bladeshape which efiects important improvements in resistance to fatiguefailure, at substantially no cost in increased rotor weight, and with animprovement in aerodynamic emciency.

While only one blade shape of this novel type has been describedspecifically, it will be apparent to those skilled in the art that manysmall changes may be made without departing from the invention, and itis intended to cover by the appended claims all such modifications asfall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An integral turbom-achine blade having the efiective length thereofsubdivided into root, intermediate, and tip portions, the root portioncomprising substantially one-third of the blade length and having asubstantially constant crescent-shaped section substantially that of animpulse t pe blade, the tip portion comprising substantially the outerthird of the blade length and having a substantially constantairfoilshaped section substantially that of a reaction type blade, theintermediate blade portion rapidly transitioning smoothly andcontinuously from g the impulse-shaped root to the reaction-shaped tip,the cross-section area of the root portion being on the order of threetimes the cross-section area of the tip portion whereby the high momentof inertia of the root section resists vibration of the blade.

2. An integral turbo-machine blade havingthe effective length thereofdivided into root, intermediate, and tip portions, the root portioncomprising substantially one-third of the blade length and having ashape substantially that of an impulse type blade, the cross-sectionshape and area of said root portion being substantially constantthroughout the radial length thereof,

the tip portion comprising substantially the outer third of the bladelength and having a substantially constant cross-section shapesubstantially that of a reaction type blade, the intermediate bladeportion rapidly transitioning smoothly and continuously from theimpulse-shaped root portion to the reaction-shaped tip portion, thecross-section area of the tip portion being substantially constant alongthe radial length thereof and being not more than about one-third thecross-section area of the root portion, while the cross-section area ofthe intermediate portion decreases rapidly from the root portion to thetip portion, whereby the tangential moment of inertia of thecross-section area is high and substantially constant throughout theroot portion, while the tip portion is of substantially smaller sectionand tangential moment of inertia.

3. In an integral turbomachine blade having the effective length thereofdivided into root, intermediate, and tip portions, the root portioncomprising substantially one-third of the blade length and having acrescent shape substantially that of an impulse type blade, thecrosssection shape and area of said root portion being substantiallyconstant throughout the radial length thereof with an entrance angle onthe order of 30 and an exit angle on the order of 30, the tip portioncomprising substantially the outer third of the blade length and havingsubstantially constant airfoil-shaped cross-section substantially thatof a reaction type blade with an entrance angle on the order of and anexit angle in the neighborhood of 20, the intermediate blade portionrapidly transitioning smoothly and continuously from the impulseshapedroot portion to the reaction-shaped tip portion, the cross-section areaof the tip portion being on the order of one-third as large as that ofthe root portion while the cross-section area of the intermediate bladeportion decreases rapidly from the root portion to the tip portion,whereby the tangential moment of inertia of the cross-section area ishigh and substantially constant throughout the root portion while thetip portion is of substantially smaller section and moment of inertia.

THOMAS N. HULL, JR.

References Cited in the file of this patent UNITED STATES PATENTS

